Among the most important selling features of an automobile is the quality of its painted finish. For this reason, automobile manufacturers are particularly concerned with producing a high quality exterior finish on today's automobile.
Many of the automobile parts currently used are fabricated from lightweight materials, such as aluminum and plastics, which do not themselves possess a desirable finish, or prematurely lose that finish when exposed to the elements. Various approaches have been taken to providing a high quality automotive paint-like finish to lightweight parts, such as molded plastic parts. One approach which has received considerable attention uses a preformed paint-like film which can be either bonded to a preformed part or applied to a plastic part as it is molded.
One such surfacing film is a paint transfer article described in U.S. Pat. No. 4,818,589. See also Johnson, B., "Development of an Automotive "Paint Film" Concept," SAE Technical Paper Series, 890532, 1989. The paint transfer articles described in the preceding references use standard autobody paints for the coating composition to add the decorative feature to a surfacing film for automobile body parts. Properties of the cited carrier restrict its utility as a protective carrier.
Commonly-owned U.S. Pat. Nos. 4,810,540, 4,931,324 and 4,943,680 describe surfacing films which are formed from liquid cast molecularly unoriented polymer films. The molecularly unoriented polymer films produce high quality, high gloss paint-like finishes on contoured articles such as automobile body parts.
The paint-like coating compositions of the foregoing surfacing films are typically formed by spraying, casting or coating the paint-like layer onto a casting base. The casting base must have uniform texture or gloss, since this texture and gloss is transferred to the surface of the colored surfacing film layer.
The casting base is selected for its stability under a variety of conditions. In general, high tensile strength is required so that the casting base, and consequently the paint-like layer, is not distorted by application of the tension required to advance the casting base through a coating machine. After application of the paint-like coating composition, the coated casting base would be typically run through a heating chamber to remove solvents or to fuse or cure/crosslink the polymeric components of the paint-like coating and so the casting base must have low extensibility at elevated temperature.
Additionally, the solvents of the coating composition may distort or impair the finish of the casting base, and so the base must be substantially solvent resistant. Accordingly, suitable films for the purposes described in the foregoing references are films that are stiff and rigid, resist distortion at elevated temperature, and are unaffected by harsh solvents.
In many applications, the casting base will be left in place on the surfacing film after the surfacing film is manufactured so that it thus serves as a protective carrier layer during shipment and handling and during subsequent application of the film to a substrate. In such applications, the carrier layer should be sufficiently flexible and extensible to conform intimately to the contours of the substrate. Such conformation may be enhanced by the application of heat, i.e. the carrier layer can be thermoformed.
The thermoformability of these carrier layers, however, is substantially limited. This is because the physical properties (e.g., low extensibility at elevated temperature) which make a film a good casting base are the very properties which would serve to restrict thermoformability if the film is to later serve as a carrier layer. Accordingly, the carrier layers of the foregoing references necessarily have restricted thermoformability. Carrier layers with restricted thermoformability may be suitably employed in applications where the amount of stretching or drawing is limited, but they produce defects such as cracking, wrinkling, crazing, and blushing when subjected to higher degrees of drawing or stretching by thermoforming and such films do not conform to replicate the mold surface. Because of the intimate proximity of the carrier layer and the paint-like layer, those defects are transferred to the paint-like layer, adversely affecting its value as a decorative feature of an article.